The JHM strain of mouse hepatitis virus (JHMV) produces an acute encephalitis accompanied by demyelination. Survivors have chronic demyelination associated with persistent viral RNA and antigen to the central nervous system (CNS). Persistence is not associated with infectious virus and no data is yet available to suggest a mechanism for the persistence of a 31 Kb viral genome. The strength of the immune response during acute infection is a critical determinant in establishing chronic disease; however, its relevance during persistence has not been established. This proposal addresses two interrelated aspects of JHMV- induced chronic disease: potential role of viral evolution leading to attenuated replication and the role of CD8+ T cell activation for disease outcome and memory. Our data indicate that genetic mutations associated with CNS persistence are not the result of immune-driven events. Therefore, mutations associated with persistence will be tested in three areas of the viral genome involved in replication: the nucleocapsid (N) and envelope (E) proteins and the leader RNA sequence. N protein is involved in transcription and translation. Defects in any of these could result in attenuated replication and persistence. The second aim examines the role of CNS cells as antigen-presenting cells of virus clearance from the CNS. However, their inability to provide sterile immunity indicates a suboptimal effector response. Characterization of the relative frequencies and phenotype associated with the cytolytic effector phase found exclusively in the CNS will provide insights into the development of effector function relative to viral replication. The third aim explores the specificities of unique memory CTL populations contained within the CNS. CTL represent in the CNS during persistent infection appear to loose the ability to recognize potential variations in the immunodominant epitope. The selection of CTL subsets exhibiting enhanced selectivity into the memory population will be monitored by phenotypic characterization and functional assays. These experiments will provide evidence for a potential effector function in response to residual antigen during viral persistence. This proposal examines the critical question of how the CNS regulated CTL induction, activation and memory responses during a parenchymal infection that results in persistent infection and ongoing chronic demyelination.